1. Field of the Invention
The present invention relates to an image processing apparatus and method for encoding image data, and to a storage medium storing processes of such a method.
2. Related Background Art
Image, particularly a multi-value image, contains a very large amount of information. The data amount required for storing or transmitting such an image is bulky. In storing or transmitting an image, therefore, high efficiency encoding is used to reduce the data amount by removing a redundancy of an image or changing the contents of an image to a degree that deterioration of the image is not visually recognizable.
However, even if the data amount can be reduced to some degree by incorporating high efficiency encoding, it takes in some cases a relatively long time to transmit or read the encoded data. It is desired to use hierarchical encoding by which a reception side receiving transmitted encoded data can recognize the outline of an image at an earlier stage of data reception and can gradually recognize a high quality image as the remaining encoded data is received.
With conventional hierarchical encoding, image data of each pixel represented by multi-value is converted into a plurality of bit planes and the bit planes are transmitted in the order from an upper level bit plane to a lower level bit plane.
For example, JPEG which was recommended by ISO and ITU-T as an international standard encoding scheme for still images, stipulates several types of encoding methods in accordance with a use objective of the contents of an image to be encoded or of the encoded data. For example, JPEG stipulates SS (Spectrum Selection) and SA (Successive Approximation) which realize hierarchical encoding during extended DCT processes.
The details of JPEG are described in ITU-T Recommendation T.81, ISO/IEC 10981-1, and so the description thereof is omitted. With SA, an image of each block is subject to discrete cosine transform (DCT), and all the frequency components are quantized into n-bit coefficients. A plurality of coefficients thus obtained are converted into n hierarchical levels (n to 1), and the bit planes are transmitted in the order from an upper level bit plane to a lower level bit plane.
With this conventional bit plane encoding method in which image data of multi-value is converted into a predetermined number of hierarchical level bit planes, and the bit planes are output hierarchically, however, there is still a redundancy in bit planes.
Also with a conventional hierarchical encoding, if the reception side receives only higher level bit planes, it is not easy to recognize the outline of an encoded multi-value image at an earlier stage of data reception.